A pulse width generator is known from U.S. Pat. No. 3,441,877, issued to Thompson, in which a measurement current I.sub.m passing through a resistor R.sub.1 obtained from a voltage source U.sub.m passes into a Miller-generator, which consists of an amplifier V, and a capacitor C connected across the input and the output of the amplifier V. A Schmitt-type trigger ST is postcoupled to the output of the amplifier V, which in turn controls a switch S of a reference voltage source Q. In one position of the switch S the input of the amplifier V is connected via a resistor R to a positive reference voltage +U.sub.r, and in another position of the switch S the input of the amplifier V is connected via the resistor R to a negative reference voltage -U.sub.r. Consequently, a reference current I.sub.r, which is superimposed upon the current I.sub.m, and is either positive or negative in dependence of the position of the switch S, passes to the input E of the Miller-type integrator M.
Across the output of the Miller-type integrator M there appears a triangular voltage U.sub.i, which according to FIG. 2, ascends or is positive-going, until it reaches an upper threshold value U.sub.s1 of the Schmitt-type trigger ST, the Schmitt-type trigger ST subsequently flipping over so as to reverse the switch S, the reference current I.sub.r changing its polarity, and the voltage U.sub.i descending, or becoming negative-going, until it reaches a lower threshold value U.sub.s2 of the Schmitt-type trigger circuit ST. At the output of the Schmitt-type trigger circuit ST, there appears a waveform of rectangular pulses, having a mark duraction T.sub.a, and a space duration T.sub.b.
The rectangular voltage waveform is governed by the equation: ##EQU1##
The ratio (R)/(R.sub.1) represents a constant of proportionality, so that R and R.sub.1 must be precision resistances.
The above U.S. Pat. No. 3,441,877 is incorporated into the present application by reference.
A pulse-time encoding apparatus is also known from U.S. Pat. No. 3,074,057 issued to Gilbert.
There is also known an integrating quasi-sweeping voltage-to-time-interval converter from volume 19, number 2, part 1 of the Russian Journal "Instruments and Experimental Techniques", March-April, 1976. The integrating quasi-sweeping voltage-to-time-interval converter described in that publication is a variable pulse width generator, including a differential amplifier with two inputs, the input voltage of the integrating quasi-sweeping voltage-to-time-interval converter being connected to one input of the differential amplifier, the other input of the differential amplifier being connected via a capacitor and via a resistor to a reference voltage source. Although the mark-to-space ratio of the integrating quasi-sweeping voltage-to-time interval converter is independent of the value of that resistor, the operation of the integrating quasi-sweeping voltage-to-time-interval converter is subject to disturbances if the input voltage is rapidly changed. The integrating quasi-sweeping voltage-to-time-interval converter is therefore only suitable for applications, where the input voltage changes only relatively slowly.